1. Field of the Invention
The present invention relates to a circuit for a loop antenna and a method for tuning.
2. Description of the Background Art
Loop antennas are known from the “Antennenbuch,” (Antenna Book) by Rothammel, Krischke, pages 67, 318, 319, 332, 333, 12th edition, 2001. Loop antennas are closed (magnetic) antennas with loop-shaped elements. With antenna loops that are small in comparison to the wavelength, they are also called frame antennas or magnetic antennas. For the frequency f=0, the impedance is also zero. For increasing frequency, the impedance behavior is inductive. This impedance behavior extends only to half-wave resonance, however, which is to say to a range of half waves, after which the impedance behavior becomes capacitive. For this reason, only small loops (up to a length of approximately a quarter wave) are called magnetic antennas. Possible geometric shapes of the loops include circular element, square element, diamond or rhombus, triangular element, rectangular element, polygonal element, and elliptical element. Small loop antennas have an inductive impedance with a first terminal and a second terminal. The inductive impedance is connected to a capacitive impedance, for example a number of capacitors (one or several), to form a resonant circuit.
Known from EP 0 663 724 B1, which corresponds to U.S. Pat. No. 5,491,715, is a method for matching the resonant frequency of an antenna resonant circuit to the output frequency of a transmitter output stage. The antenna resonant circuit contains a tuning circuit that can be operated in such a manner that it modifies the resonant frequency of the resonant circuit. The antenna resonant circuit has a phase correlation signal at its output. The phase of the phase correlation signal with respect to the supplied signal has a known relationship to the frequency difference between the resonant frequency and the supplied signal. The supplied signal and the phase correlation signal are received in a phase comparator. The resonant frequency of the resonant circuit is established as the response to the received output signal of the phase comparator.